PANGAEA

(from Greek pangaia, all earth), hypothetical protocontinent proposed by the German meteorologist Alfred Wegener in 1912 as a part of his theory of continental drift (q.v.). According to this theory, Pangaea was composed of continental sial (granitic rock), which was balanced isostatically in a layer of denser material (basalt), called sima, constituting the uppermost portion of the Earth's mantle. The protocontinent supposedly covered about half the Earth and was completely surrounded by a world ocean called Panthalassa. Late in the Triassic Period (245 to 208 million years ago), Pangaea began to break apart. Its segments Laurasia (composed of all the present-day northern continents) and Gondwanaland (all of the present southern continents) gradually receded, resulting in the formation of the Atlantic Ocean. The breakup of Pangaea is now explained in terms of plate tectonics (q.v.). This theory states that the Earth's outer shell, or lithosphere, consists of large, rigid plates, which move relative to each other and interact at their margins, where they diverge, converge, or slip past one another. Pangaea split apart at one of the divergent plate boundaries, and a rift developed beneath the continent. As the two segments of the continent pulled farther apart, molten rock material from the asthenosphere, the layer underlying the lithosphere, flowed upward to fill the void, creating the floor of the new Atlantic Ocean basin.